The purpose of a vehicular transmission is to provide a neutral, one or more forward and reverse driving range that impart power from engine, and/or other power sources, to the drive members which deliver the tractive effort from the vehicle to terrain over which the vehicle is being driven. As such, the drive members may be front wheels or rear wheels.
Although a wide variety of transmission types are available, transmissions adapted to receive the output power from either an engine or an electric motor, or both, can be provided that will operate at high efficiencies over a wide variety of operating conditions. Fuel economy is critical in a hybrid electric vehicle. Spinning the planetary gear sets and wet clutch plates at all times consume energy due to friction losses, which negatively affects fuel economy. The transmission of a hybrid electric vehicle includes an engine, the first and second motor/generators and energy storage device, such as batteries, wherein the engine and motors can drive the wheels individually, the engine can charge the energy storage device through the electric machine operating as a generator, and vehicle kinetic energy can be recovered and regenerated using the wheel brakes to drive the generator and recharge the energy storage device. A control unit regulates power flow among the energy storage device and the motor/generators as well as between the first and second motor/generators.
The transmission for a hybrid electric vehicle, specifically, a plug-in hybrid electric vehicle, and the energy storage device is significantly increased in capacity so that electrical energy from the electric grid can be used of electric drive is expected. As a result, a much greater use of electric drive is expected. The transmissions adapted to receive the output power from either the first or second motor/generators, or both, would to be provided that will operate at high efficiencies over a wide variety of operating conditions. The direct connection of the first motor/generator to the engine shaft causes the first motor/generator shaft to rotate as the engine driving the wheels individually. The direct connection of the second motor/generator to the wheel speed causes the second motor/generator shaft to rotate as the vehicle moves when the engine is off. These cause several issues including (i) as the vehicle increases in speed in the engine mode, the first and second motor/generators speed get excessively high causing a durability concern for the bearing, planetary gear sets, the first and second motor/generators; (ii) Spinning the planetary gear sets and wet clutch plates at all times consume a lot of energy due to friction losses causing driving efficiency dropped dramatically; (iii) since the first and second motor/generators are not being used, them generate an unnecessary spin loss; (iv) lowering of available torque needed to start the engine; (v) the transmission is short of fixed ratios causing the inefficient operation of the engine and the motor/generators.
Various control schemes and operational connections between the various components of the hybrid electric system are known, and the control system must be able to engage and disengage the various components from the transmission in order to perform the functions of the hybrid electric system. Engagement and disengagement are known to be accomplished within the transmission by employing selectively operable clutches. Clutches are known in a variety of designs and control methods. One known type of clutch within the transmission is a mechanical clutch operating by separating or joining two connective surfaces, for instances, wet clutch plates, operating, when joined, to apply frictional torque to each other. Engagement operation while engaged operation, and disengagement operation while disengaged operation are all clutch states that must be managed in order for the vehicle to operate properly and smoothly. Therefore, slip, or relative rotational movement between the connective surfaces of the clutch when the clutch connective surfaces are intended to be synchronized and locked, occurs whenever reactive torque applied to the clutch exceeds actual capacity torque created by applied clamping force. These cause several issues including (i) slip in the transmission results in a lot of heat and dust causing degradation for the performance and life of the transmission; (ii) resistance force between two connective surfaces for wet clutch plates becomes larger as the input member speed becomes higher resulting in a large amount of heat and a decrease in efficiency for the transmission; (iii) the oil roads of clutches are very complex and precise resulting in a significant increase in costs; (iv) once the clutches are damaged, the transmission must be disassembled for repair or replacement; (v) lubricating oil selection for the transmission must take into account clutch performance and transmission life, such as gears and bearings, it is difficult to give full play to the best performance of lubricating oil.